Conventional golf balls can be divided into two general types or groups: two-piece balls and wound balls (also know as three-piece balls). The difference in play characteristics resulting from these different types of constructions can be quite significant.
Balls having a two-piece construction are generally most popular with the average recreational golfer because they provide a very durable ball while also providing maximum distance. Two-piece balls are made with a single solid core, usually made of a crosslinked rubber, which is encased by a cover material. Typically the solid core is made of polybutadiene which is chemically crosslinked with zinc diacrylate and/or similar crosslinking agents and is covered by a tough, cut-proof blended cover. The cover is generally a material such as SURLYN.RTM., which is a trademark for an ionomer resin produced by DuPont. The combination of the core and cover materials provide a "hard" ball that is virtually indestructible by golfers. Further, such a combination imparts a high initial velocity to the ball which results in improved distance. Because these materials are very rigid, two-piece balls have a hard "feel" when struck with a club. Likewise, due to their hardness, these balls have a relatively low spin rate which provides greater distance.
At the present time, the wound ball remains the preferred ball of the more advanced players due to its spin and feel characteristics. Wound balls typically have either a solid rubber or liquid center core around which many yards of a stretched elastic thread or yarn are wound. The wound core is then covered with a durable cover material such as a SURLYN.RTM. or similar material or a softer cover such as Balata or polyurethane. Wound balls are generally softer and provide more spin, which enables a skilled golfer to have more control over the ball's flight and final position. Particularly, with approach shots into the green, the high spin rate of soft, wound balls enables the golfer to stop the ball very near its landing position.
Golf ball design and technology have now advanced to the point that the United States Golf Association (USGA), the organization that sets the rules of golf in the United States, has instituted a rule that prohibits the competitive use in any USGA sanctioned event of a golf ball that can achieve an initial velocity of 76.2 meters per second (m/s), or 250 ft/s plus an allowed tolerance of two percent when struck by a driver with a velocity of 39.6 m/s, i.e., 130 ft/s (referred to hereinafter as "the USGA test").
Manufacturers place a great deal of emphasis on producing golf balls that consistently achieve the highest possible velocity in the USGA test without exceeding the limit, which are available with a range of different properties and characteristics, such as spin, compression, "click," and "feel." Thus, a variety of different balls is available to meet the needs and desires of a wide range of golfers.
Regardless of the form of the ball, players generally seek a golf ball that delivers maximum distance, which requires a high initial velocity upon impact. Therefore, in an effort to meet the demands of the marketplace, manufacturers strive to produce golf balls with initial velocities in the USGA test that approximate the USGA maximum of 77.7 m/s or 255 ft/s as closely as possible.
Therefore, golf ball manufacturers are continually searching for new ways in which to provide golf balls that deliver the maximum performance for golfers of all skill levels, and seek to discover compositions that provide the performance of a high compression ball with lower compression. At the same time, external factors such as production cost and environmental concerns play a role in the design of balls.
The physical characteristics of a golf ball are determined by the combined properties of the core, any mantle layers, and the cover. These, in turn, are determined by the chemical compositions of each. With regard to the core, a number of polymers, such as polybutadiene, natural rubber, styrene butadiene, and isoprene, are commonly used in fabricating golf ball cores. Today, golf ball cores are predominantly made of polybutadiene. Moreover, in order to obtain the desired physical properties for golf balls, manufacturers have added cross-linking agents, such as metallic salts of an unsaturated carboxylic acid. The amount of cross-linking agent added is typically about 20 to 50 parts per hundred parts of polybutadiene. Most commonly, zinc diacrylate or zinc dimethacrylate are used for this purpose. Of these two cross-linkers, zinc diacrylate has been found to produce golf balls with greater initial velocity than zinc dimethacrylate.
Typically, about 5 to 50 pph (parts per hundred) of zinc oxide (ZnO) is also added to the composition. This material serves as both a filler and an activation agent for the zinc diacrylate/peroxide cure system. The zinc diacrylate/peroxide cure system, which is well known to those of ordinary skill in this art, cross-links the polybutadiene during the core molding process. The high specific gravity of zinc oxide (5.57) can serve the dual purposes of adjusting the weight of the golf ball, in addition to acting as an activation agent.
Because zinc oxide is environmentally regulated, it would be advantageous to eliminate or at least substantially reduce the amount of this material from the manufacturing process. However, when the zinc oxide is eliminated from the composition described above, there is a reduction in cure enhancement, which results in less cross-linking and a corresponding reduction in compression and velocity. This result provides a ball with a softer feel, and allows less skilled golfers to compress the ball fully, but the resulting ball has less than the maximum velocity allowed by the USGA standard.
Therefore, it would be advantageous to provide a golf ball core composition with an activation agent other than zinc oxide, i.e., wherein all or at least some of the zinc oxide commonly present was eliminated, which would, as noted above, provide a ball with a lower compression while maintaining the velocity and distance of a high compression ball. The present invention provides such a golf ball core.
The performance of the golf ball--its spin and feel characteristics, for example--is not dictated solely by the chemical properties of the core. Those of the cover and mantle layers are also of fundamental importance. The cover, however, must also provide adequate durability. Consequently, design of a golf ball cover must take into account two factors: 1) its effect on the overall feel and play of the ball, and 2) its ability to withstand stress without cutting or cracking.
Balata was the standard cover stock material until the middle 1960's when E.I. DuPont de Nemours and Co. discovered a new species of resins known as ionomers or ionomer resins. These resins are sold under the trademark SURLYN.RTM. and, to a large extent, have replaced balata as a cover stock material. Balata is now primarily reserved for the high performance, wound balls.
Chemically, SURLYN.RTM. ionomer resins are a copolymer of an olefin and an alpha, beta ethylenically unsaturated carboxylic acid with 10-90% of the carboxylic acid groups being neutralized by a metal ion; see U.S. Pat. No. 3,264,272. Today, the only commercially available ionomer resins are copolymers of ethylene and methacrylic or acrylic acid. Conventionally, these ionomer resins are distinguished by the type of metal ion, the amount of acid, the degree of neutralization, and the melt index. By varying these parameters, the resiliency, the flexural modulus and the hardness of the ionomer resins can be varied significantly. Thus, golf balls can be made having significantly different properties and performance using different ionomers or ionomer blends.
Dunlop Rubber Company obtained the first patent on the use of Surlyn for the cover of a golf ball, see U.S. Pat. No. 3,454,280 issued Jul. 8, 1969. Since then, there have been a number of disclosures on the use of these ionomer resins in the cover composition of a golf ball. See, for example, U.S. Pat. Nos. 3,819,768 issued Jun. 25, 1974; 4,323,247 issued Apr. 6, 1982; 4,526,375 issued Jul. 2, 1985; 4,884,814 issued Dec. 3, 1989; and 4,911,451 issued Mar. 27, 1990.
In November of 1986, DuPont introduced sodium and zinc ionomer resins having a low flexural modulus, below about 20,000 psi.sup.1 (the so-called "low modulus" ionomers), and suggested using and blending the same with standard sodium ionomer resins for making a golf ball cover. Golf ball covers made with greater than 50% of the low modulus ionomers are relatively soft and form golf balls having high spin properties. FNT .sup.1 Flexural modulus as published by Dupont.
In December of 1986, DuPont introduced a lithium ionomer resin which, together with standard sodium and zinc ionomers existing before November 1986, comprises a group of ionomers known as the "conventional" ionomers. The lithium ionomer as a copolymer of ethylene and methacrylic acid, optionally containing a softening acrylate comonomer. These lithium ionomer resins typically have a flexural modulus of about 60,000 to 70,000 psi (9,415 MPa to 10,980 MPa)..sup.2 DuPont suggested that lithium ionomer resins could be used to produce a golf ball cover which would be more cut resistant and harder than a cover made with either sodium or zinc ionomer resins. DuPont also suggested that a golf ball having a cover made from a lithium ionomer resin would go farther, have a higher coefficient of restitution and be less prone to cutting (i.e. more durable) than a golf ball made from other known ionomer resins, e.g. sodium and zinc ionomer resins and blends thereof. DuPont further suggested that lithium ionomer resins could be used in blends with low modulus sodium ionomer resins where they can impart better cut resistance to these other materials. FNT .sup.2 Flexural modulus as published by DuPont.
U.S. Pat. No. 5,000,459 discloses the use of lithium ionomers and sodium ionomers. More particularly, the reference discloses that balls made from at least 50% of lithium ionomer resin having 5 to 20 parts by weight of acrylic or methacrylic acid have excellent durability.
It has also been found that covers comprising about a 50/50 blend of a sodium ionomer having about 19% by weight methacrylic acid (a so-called "high acid" ionomer resin) and a lithium ionomer having about 15% by weight methacrylic acid are advantageous for distance or low spin type balls. These covers have a high resiliency, high flexural modulus and high hardness.
Despite these innovations, the golf ball covers they have provided still fail to interact with core and mantle layers to provide balls that combine the control and feel of a wound balata ball with the durability of a solid, two-piece ball. The present invention moves the art closer to that goal. It provides durable covers that interact with the cores disclosed herein to provide balls with desireable spin, compression, "click," and "feel."